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Filtering out common-mode noise in congested automotive environments

Feature

By Jeff Elliott, US-based technical writer

There are many factors adding to interference in electronics, disturbing their functionality and even damaging the devices. Tightly packing components into ever-shrinking spaces makes the situation worse, leading to signals interfering with each other.

Today’s automobiles are a prime example of growing electronics into smaller spaces: Wi-Fi, Bluetooth, satellite radio, GPS systems, LED lights, cameras and many instruments on board are growing in number, as do numerous other systems that use DC motors.

Prevention is better than cure

To eliminate unwanted noise, the industry typically uses shielding, along with EMI filters in various configurations. But some of these traditional solutions are no longer sufficient, given increases in operating circuit frequency and higher frequency noises that expand the affected frequency range. In addition, a growing number of electronic devices are more easily affected by noise, even by weak energy fields, since many devices nowadays operate at lower voltages. Hence, OEMs are abandoning options such as two-capacitor differentials, three-capacitor (one X-cap and two Y-caps) solutions, feedthrough filters, common-mode chokes or combinations of these, for more suitable solutions such as monolithic EMI filters that deliver superior noise suppression in substantially smaller packages.

EMI/RFI filters

Strong electromagnetic fields can cause unwanted currents, causing interference or circuit malfunction.

EMI/RFI can be conducted or radiated. In conducted EMI, noise travels along the conductors, whereas in radiated, noise travels through air as magnetic fields or radio waves.

Even if the applied energy is weak, it can mix with radio waves, causing reception loss, abnormal noise in sound or disrupted video.

Sources of noise can be natural, such as electrostatic discharge or lighting, or artificial, such as contact noise, high-frequency device leakage, unwanted emissions (e.g. harmonic emission from digital circuits, switching power supplies emissions), and others. Noise can also be generated from inside the circuit of an electronic device, to cause interference with another in the same space.

Usually, EMI/RFI noise is common-mode noise, so the solution to eliminating unwanted high frequencies is an EMI filter, either as a separate or embedded device. This also helps OEMs meet regulatory standards that exist in most countries, which limit the amount of emitted noise.

EMI filters normally comprise passive components, such as capacitors and inductors.

“The inductors allow DC or low-frequency currents to pass through, whilst blocking the harmful unwanted high-frequency currents. The capacitors provide a low impedance path to divert high-frequency noise away from the filter’s input, either back into the power supply or to ground,” said Christophe Cambrelin of Johanson Dielectrics, a company that manufactures multi-layer ceramic capacitors (MLCC) and EMI filters.

Traditional common-mode filtering approaches include low-pass filters comprised of capacitors that pass signals with a frequency lower than a selected cutoff frequency and attenuate signals with frequencies higher than the cutoff.

A common starting point is to apply a pair of capacitors in a differential configuration, with one capacitor between each trace and ground of the differential input. The capacitive filter in each leg diverts EMI/RFI to ground above a specified cutoff frequency. Because this configuration involves sending opposite-in-phase signal through two wires, the signal-to-noise ratio is improved, with the unwanted noise sent to ground.

“Unfortunately, the capacitance value of an MLCC with X7R dielectric (typically used for this function), varies significantly with time, bias voltage and temperature. So, even if the two capacitors are tightly matched at room temperature, with low voltage, at a given time it’s very likely they’ll end up with a very different value, once voltage or temperature has changed. This mismatch between the two lines will cause unequal response near the filter cutoff, therefore converting common-mode noise to differential noise,” said Cambrelin.

Another solution is to bridge a large value X capacitor across the two Y capacitors. The X capacitor shunt delivers the desired effect of common-mode balancing, although there’s differential signal filtering as undesired side effect.

Perhaps the most common solution and an alternative to low-pass filters is the common-mode choke – a 1:1 transformer where both windings act as primary and secondary windings. In this approach, current through one winding induces an opposing current in the other. Unfortunately, they are also large, heavy, expensive devices and subject to vibration-induced failure.

Still, an ideal common-mode choke with perfect matching and coupling between the windings is completely transparent to differential signals, and presents very high impedance to common-mode noise.

One disadvantage of common-mode chokes is the limited frequency range due to parasitic capacitance. For a given core material, the higher the inductance used to obtain lower frequency filtering, the greater the number of turns required, leading to consequent parasitic capacitance that defeats high-frequency filtering.

Mismatch between windings from mechanical manufacturing tolerance can cause mode conversion, where a percentage of the signal converts to common-mode noise, and, vice versa, giving rise to electromagnetic compatibility and immunity issues and ineffective inductance in each leg.

However, a major advantage of common-mode chokes is that differential signals (to pass) operate in the same frequency range as the common-mode noise that must be suppressed. With a common-mode choke, the signal passband can extend into the common-mode reject band.

Monolithic EMI filters

Despite the popularity of common-mode chokes, a better alternative may be monolithic EMI filters. When properly laid out, these multilayer ceramic components provide superior rejection of common-mode noise. They combine two balanced shunt capacitors in a single package, with mutual inductance cancellation and shielding effect. Johanson Dielectrics’s monolithic filters use two separate electrical pathways within a single device, attached to four external connections.

To avoid confusion, it should be noted that a monolithic EMI filter is not a traditional feedthrough capacitor. Although the two filters look identical (same package and external look), their design and connections are very different.

Like other EMI filters, monolithic EMI filters attenuate all energy above a specified cutoff frequency and only pass certain signals whilst diverting unwanted noise. The key, however, is their very low inductance and matched impedance. With monolithic EMI filters, the terminations connect internally to a common reference (shield) electrode within the device, separating the plates. Electrostatically, the three electrical nodes are formed by two capacitive halves that share common reference electrodes, all within a single ceramic body.

“Being very well-balanced, a monolithic EMI filter introduces almost no conversion of common-mode noise to differential signals, or vice versa. Furthermore, having a very low inductance makes it particularly effective at high frequencies,” said Cambrelin.

The balance between capacitor halves also means piezoelectric effects are equal and opposite, canceling out. This also affects temperature and voltage variations, so components age equally on both lines.

“Compared to common-mode choke solutions, this device provides significantly more RFI suppression in a substantially smaller package. It also rejects a much wider frequency band,” added Cambrelin.

If there’s a downside to monolithic EMI filters, it is that they can’t be used if the common-mode noise is at the same frequency as the differential signal. In those cases common-mode chokes are a better solution, says Cambrelin.

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